Adeno-associated virus (AAV) vector has been used in over 100 clinical trials with promising results, in particular, for the treatment of blindness and hemophilia B. AAV is non-pathogenic, has a broad tissue tropism, and can infect dividing or non-dividing cells. More importantly, AAV vector transduction has induced long-term therapeutic transgene expression in pre-clinical and clinical trials. Currently there are 12 serotypes of AAV isolated for gene delivery. Among them, AAV8 has been shown to be the best for mouse liver targeting. Extensive studies in pre-clinical animals with FIX deficiency and Phase I/II clinical trials have been carried out using AAV2 and AAV8 in patients with hemophilia B. The results from these trials are very promising; however, the FIX expression from patients receiving AAV/FIX was not proportional to what has been achieved in animal models even though the same vector dosage/kg was used. When 1×1011 particles of AAV8 encoding FIX were used in FIX knock out mice for systemic administration, 160% of normal level FIX was detected in blood. However, when 2×1011 particles of AAV8/FIX were administered, only 40% of FIX was achieved in primates and less than 1% of FIX was found in human. The inconsistent FIX expression following AAV vector transduction among these species may be due to altered hepatocyte tropism in different species. Another interesting finding from AAV FIX clinical trials is the capsid specific cytotoxic T lymphocyte (CTL) response that eradicates AAV transduced hepatocytes, resulting in therapeutic failure. This phenomenon has not been seen in animal models following AAV delivery, which points out another variation between preclinical and clinical studies. When a much higher dose of AAV/FIX vector was used, FIX expression was detected in both clinical trials using either AAV2 or AAV8; however the blood FIX level decreased at week 4 or 9 post injection, respectively. Further studies suggested that AAV vector infection elicited a capsid specific CTL response, which appeared to eliminate AAV transduced hepatocytes. Therefore, the results from these clinical trials highlight the necessity to explore effective approaches for enhancement of AAV transduction without increasing vector capsid burden. Any vector improvement that reduces AAV capsid antigen effect will also impact the daunting vector production concerns and be a welcome addition to viable gene therapy drug development.
Adeno-associated virus (AAV), a non-pathogenic-dependent parvovirus that needs helper viruses for efficient replication, is utilized as a virus vector for gene therapy because of its safety and simplicity. AAV has a broad host and cell type tropism capable of transducing both dividing and non-dividing cells. To date, 12 AAV serotypes and more than 100 variants have been identified. Different serotype capsids have different infectivity in tissues or culture cells, which depend on the primary receptor and co-receptors on the cell surface or the intracellular trafficking pathway itself. The primary receptors of some serotypes of AAV have been determined, such as heparin sulfate proteoglycan (HSPG) for AAV2 and AAV3, and N-linked sialic acid for AAV5, while the primary receptor of AAV7 and AAV8 has not been identified. Interestingly, AAV vector transduction efficiency in cultured cells may not always be translated into that in animals. For instance, AAV8 induces much higher transgene expression than other serotypes in mouse liver, but not in culture cell lines.
Of the above-mentioned 12 serotypes, several AAV serotypes and variants have been used in clinical trials. As the first characterized capsid, AAV2 has been most widely used in gene delivery such as RPE 65 for Leber congenital amaurosis and Factor IX (FIX) for hemophilia B. Although the application of AAV vectors has been proven safe and therapeutic effect has been achieved in these clinical trials, one of the major challenges of AAV vector is its low infectivity that requires relatively huge numbers of virus genomes. AAV8 vector is another vector which has been used in several clinical trials in patients with hemophilia B. The results from AAV8/FIX liver-targeted delivery have demonstrated that there are distinct species-specific differences in transgene expression between mice, non-human primates and humans. While 1010 vg of AAV8 with FIX gene could reach supra-physiologic levels (>100%) of FIX expression in FIX knock-out mice, only high doses (2×1012 vg/kg of body weight) could induce detectable FIX expression in humans. Based on these results described above, the development of effective strategies to enhance AAV transduction is still necessary.
The majority of people have been naturally exposed to AAVs. As a result, a large portion of the population has developed neutralizing antibodies (Nabs) in the blood and other bodily fluids against certain serotype AAVs. The presence of Nabs poses another major challenge for broader AAV applications in future clinical trials. Many approaches have been explored to enhance AAV transduction or evade Nab activity, especially genetic modification of the AAV capsid based on rational design and directed evolution. Although several AAV mutants have demonstrated high transduction in vitro or in animal models, along with the capacity to escape Nabs, the modification of the capsid composition provides an ability to alter the cell tropisms of parental AAVs.
The present invention addresses a need in the art for AAV vectors with combined desirable features.